Sally is talking with her friend, Don. 
S: Tell me, Don. How come you're from Spain but you can't speak Spanish very well? 
D: It's not my mother tongue. My father is Spanish and my mother is American. We usually speak English at home. 
S: Oh, I see. 
D: Your Spanish is probably better than mine. 
S: Yes, but you speak English more fluently than I. 
D: How come? Aren't you American? 
S: Yes, but my mother is Spanish. 
D: Don't tell me... 
S: That's right. We usually speak Spanish at home. 
D: It doesn't matter as long as we can communicate. 
S: That's true.
mother tongue = mother language = native language
My father is Spanish and my mother is American.  Spanish,American是adj.
or: My father is a Spanish/Spaniard and my mother is an American.  n.
spinach 菠菜
Betty bettered her performance by studying hard.   better vt.提升
He is a fluent speaker of English. = He is a good command of English.
A friend in need is a friend indeed.
Don't tell me. = I don't believe it.   coincidence